1. Field
The present invention relates to the field of thermal processing for semiconductor and micromachined substrates, and, in particular, to determining the temperature of a processing chamber by measuring light intensity
2. Related Art
Rapid thermal annealing (RTA) is commonly performed using lamps. The workpiece, for example, a silicon wafer, a micro-machine or other device is exposed to the heat lamps for a short period of time to heat it. The workpiece is then pulled away or the lamps are turned off to allow the workpiece to cool. The temperature, timing and any repetitions for an annealing process are carefully measured and controlled so that the intended effect can be obtained reliably and repeatedly.
Since the lamps primarily operate by exposing the workpiece to infrared light, photodetectors are often used to measure the light output and the temperatures. Photodetectors have the advantage of being quick and accurate as compared to thermocouples and other types of temperature sensors. For many processes, photodetectors are used in combination with thermocouples and other temperature sensors to control the lamps, the timing, and other aspects of the annealing process.
For some workpieces, such as a silicon wafer, the infrared light from the heat lamps is absorbed by the silicon and other components in the wafer. As the wafer heats up, it begins to radiate light that is characteristic of it's own temperature, which is separate from the temperature of the lamps or the chamber in which the workpiece is being heated.
In many RTA tools, the temperature sensor inside the tool is pointed at the workpiece in order to measure the infrared light radiated by the workpiece. One common temperature sensor used in RTA tools is a pyrometer. The primary detector in the pyrometer is a photodiode which receives light radiated by the workpiece. The intensity of this light is then converted into a temperature measurement. A primary difficulty in accurately measuring the temperature of the workpiece is the influence of infrared light from other sources that are present in the chamber. These other sources include light radiating off the walls of the processing chamber inside the tool as well as light that comes directly, or via reflections, from the lamps.
A variety of approaches have been used to limit the effect of these other light sources. One common approach is a long light tube such as an optic fiber which is placed close to the workpiece to prevent other sources of light from entering the optic fiber. Another approach is to place the heat source on one side of the workpiece and the pyrometer on the opposite side of the workpiece. A further approach is to cycle the heat lamps on and off and only measure the temperature while the heat lamps are off. More complex methods make comparisons of the intensity when the heat lamps are on versus when the heat source is off. Another approach is to carefully choose the heat source so that it only produces light that is not detected by the photo detector. An alternative approach is to alter the light source so that certain wavelengths are absent in the spectral output of lamps.
RTA has become popular for a wide range of different processes. These processes can be applied to silicon substrates, gallium arsenide substrates, photocell manufacture, micromachines, and a wide range of other processes. RTA is used for contact alloying, ion implantation, silicide formation, nitridation of metals, oxidation processes, reflowing glass structures, oxidation at high temperatures, high and low k annealing, and annealing of copper and other metal structures. RTA is also used with chemical vapor deposition (CVD).
In many of these processes, the workpiece such as a silicon substrate or solar panel is exposed to temperatures above 200° C., which may be anywhere from 200° C. to 1500° C. for a short amount of time such as a few minutes or a few seconds. The workpiece is then brought back to ambient temperature and the process is repeated. Because the workpiece is exposed to the high temperature for only a short amount of time and because the amount of thermal exposure is important to the process, the temperature within an RTA tool is controlled very precisely.